Flexural vibrator as resonator for mechanical filters

ABSTRACT

In a mechanical filter employing flexural vibrators as resonators, such a vibrator undergoes flexural vibrations. Each vibrator is mounted within the filter arrangement by one or more holding elements. The holding elements are connected to the respective vibrator at a position spaced from the vibration nodes of the vibrator and are designed so as to constitute a quasi (2n1/4) lambda flexural coupling line for the flexural vibrations established in the vibrator. The cross section of both the vibrators and the holding elements can be circular.

United States Patent [191 Schussler Mar. 5, i974 22 Filed:

[ FLEXURAL VIBRATOR AS RESONATOR FOR MECHANICAL FILTERS [751 lnvemofl e ayss i len i int mqbe. .t

Germany [73] Assignee: LICENTIA Paterit-Verwaitungs-G.m.b.H., Frankfurt am Main, Germany Dec. 27, 1972 [2]] Appl. N0.: 318,935

[30] Foreign Application Priority Data Dec. 30, 1971 Germany P 21 65 552.7

[52] US. Cl. 333/711, 333/30 R [51] Int. Cl. H0311 9/06, H03h 9/24 [58] Field of Search 333/71, 72, 30 R, 30 M [56] References Cited FOREIGN PATENTS OR APPLICATIONS 1,541,982 4/1970 Germany 333/71 Primary Examiner-Eli Lieberman Assistant ExaminerMarvin Nussbaum Attorney, Agent, or FirmSpencer & Kaye [5 7] ABSTRACT 5 Claims, 7 Drawing Figures FLEXURAL VIBRATOR AS RESONATOR FOR MECHANICAL FILTERS 7 BACKGROUND OF THE INVENTION The present invention relates to mechanical filters, particularly of the type composed of a series of prismatic resonators which are arranged so that their longitudinal axes are parallel to one another and connected together by means of mechanical coupling elements.

In the prior art, different types of controls have been employed for the resonators, and attempts have been made to construct resonators having different crosssectional shapes, each of which is preferably suited for a specific type of vibration and suppresses other types of vibration. It has been found to be advantageous to construct flexural vibrators with a rectangular cross section but not a square cross section. The drawback of such a requirement for a rectangular cross section arises due to the nature of the cold working process used in shaping the material for these resonators. The specific type of cold working process that is preferable for this purpose is a drawing process which facilitates the production of resonators having circular cross sections, whereas the fabrication of rectangular cross sections with arbitrary cross-sectional dimension ratios becomes very complicated.

In order to simplify the production of such resonators it has been necessary to make some compromises re garding the desired results. It has been proposed to fab.- ricate flexural resonators from cylindrical rods and to form on the rod one or a plurality of planar surfaces parallel to the longitudinal axis of the rod-shaped resonator by the removal of material. An example for such a proposal is disclosed in the German Offenlegungsschrift (Laid open Application) No. 1,541,975. The resonators formed in this manner exhibit a preferred flexural direction but it is not possible to sufficiently attenuatc flexural vibrations occurring in a direction perpendicular to this preferred direction. Furthermore, although much less material need be removed to produce a resonator of this shape than would be required to produce a resonator having the more favorable rectangular cross section, the processes for shaping the resonator in this manner are still relatively complicated.

Consequently, there still existed, in the prior art, the problem of providing a flexural vibrator with a preferred flexural direction which could be easily manufactured.

In such prior known flexural vibrators, one common characteristic feature is the holding elements which are connected at the vibration nodes. These holding elements are usually provided in the form of rods mounted on a base plate. When the flexural vibrator is vibrated, 'the rods experience torsional vibrations substantially SUMMARY OF THE INVENTION Anobject of the present invention is to provide a resonator which can be easily constructed while avoiding the drawbacks discussed above.

A further object of the present invention is to construct a resonator which can have a circular cross section and is mounted in such a manner that undesirable vibrations are attenuated.

The flexural vibrator constructed in accordance with the present invention, which is used as a resonator for mechanical filters, is connected with a base plate by one or more holding elements. Each holding element is connected to the resonator at a substantial distance from the vibration nodes and is formed so as to constitue a quasi (2nl/4))t flexural coupling line for the flexural vibrations established within the vibrator in the desired vibration direction of the vibrator perpendicular to the longitudinal axis of the holding element at the center transmission frequency of the filter.

The term quasi (2nl/4)1\ coupling line, or the /\/4 type coupling is discussed in the paper by M. Borner and H. Schussler, Mechanische Filter fuer Zwischenfrequenzen [Mechanical filters for intermediate frequencies] published in the book Microminiaturisation in Automatic Control Equipment and in Digital Computers edited by J. Berghammer and published by Verlag Oldenbourg, Munich, 1966, pages 717-730, particularly pages 722 and 723.

The holding element substantially acts as a longitudinal coupler for the undesirable flexural vibrations of the flexural vibrator in a direction parallel to the longitudinal axis of the holding element and perpendicular to the base plate. The high coupling factor with respect to the base plate which is exhibited by the holding element provides for detuning and strong attenuation of the undesirable vibration. The high coupling factor is obtained if the length of the holding elements is less than one-quarter of the wavelength of the longitudinal vibrations of the holding elements at approximately the center transmission frequency of the filter.

The flexural vibrator according to the present invention exhibits a preferential vibration direction, which is the desired direction of vibration, and detunes and attenuates any vibrations in the undesired direction substantially independently of the cross-sectional shape of the flexural vibrator. The vibrator, therefore, can have a circular cross section, which is desirable for manufacturing purposes/A square cross section, which is also considered undesirable in conventional flexural vibrators can also be used in accordance with the present invention.

The holding elements also are preferably constructed with a circular cross section in order to facilitate their fabrication.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. la is a front elevational view of a resonator mounted in accordance with prior known techniques and showing the desired torsional vibrations produced within the holding elements.

FIG. lb is a top plan view of the embodiment shown in FIG. la.

FIG. 10 is a view similar to FIG. 1a but illustrating the occurrence of an undesirable vibration in the vibrator.

FIG. 2a is a front elevational view of a resonator mounted in accordance with the present invention.

FIG. 2b is a top plan view of the embodiment shown in FIG. la.

FIG. 2c is a view similar to FIG. 2a but illustrating the occurrence of an undesirable vibration in the vibrator.

FIG. 2d is a side view of the embodiment shown in FIG. 2a, taken in the direction of the arrow II of FIG. 20.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIGS. la-c, a flexural vibrator l is mounted in a known manner on a base plate 3 by holding elements 2. The desired vibration direction 6 (FIG. 1b) is perpendicular to the longitudinal axis of the flexural vibrator 1 and parallel to the base plate 3. This de sired vibration establishes the vibration nodes identified by the dot-dash lines 4. In addition to the flexural vibrations in the desired direction, the occurrence of other undesirable flexural vibrations is possible in a direction perpendicular to the longitudinal axis of the flexural vibration l inclined with respect to the base plate 3. An example of such vibration is shown in FIG. 1c in which the flexural vibrator I vibrates in the undesirable direction 7 perpendicular to the base plate 3 and with vibration nodes 5.

In the conventional flexural vibrator arrangement as shown in FIG. I, the holding elements 2 are disposed coaxially with the node lines 4 associated with the desired vibrations 6. As long as the vibrator l of FIG. 1 vibrates exclusively in the desired direction 6, the holding elements 2 experience torsional vibrations 8 (FIG. Ia). Flexural vibrations of resonator 1 in the undesirable direction 7 lead to flexural vibrations 9 (FIG. 10) ofthe holding elements in the direction along the longitudinal axis of the resonator 1. Since the holding elements 2 are dimensioned so that their torsional vibrations 8 only slightly attenuate the vibrations of resonator I in the desired direction 6, the holding elements 2 also produce only a slight attenuation of the vibrations in the undesirable direction 7 even with the occurrence of the flexural vibrations 9.

In contrast to the conventional mounting shown in FIG. I, the holding elements 2 in accordance with the present invention as shown in FIG. 2 are connected to the flexural vibrator at positions offset from the node lines 4, but are oriented parallel thereto. Their precise spacing from, and the direction in which they are spaced from, the node lines has no significant influence on the achievement of the desired results.

As long as resonator l of FIG. 2 vibrates exclusively in the desired direction, the holding elements 2 undergo flexural vibrations 10 (FIG. 2d) perpendicularly to the longitudinal axis of the resonator l and approximately parallel to the base plate 3. Flexural vibrations of resonator l in the undesirable direction 7 lead to longitudinal vibrations 11 (FIG.

Since placing of the holding elements 2 of FIG. 2 outside of the node lines 4 will influence the natural frequency ofthe vibratory system consisting of these holding elements 2 and the actual resonator 1, it becomes desirable to design the holding elements so that they constitute quasi 2n-l/4A flexural coupling lines for the flexural vibrations established in the vibrzir l elements to experience longitudinal vibrations 11, the holding element 2, designed in accordance with the above equation, exhibits a substantially rigid coupling to the base plate 3 (FIG. 2) thus producing a strong attenuation of the undesirable vibrations. This high coupling factor can be obtained if the holding elements are designed with a length less than one-quarter of the wavelength of the longitudinal vibrations of the holding elements at approximately the center transmission frequency of the filter.

In a practical filter with a center frequency of about 50 kc/s and a total bandwidth of 3.6 kc/s the vibrators l have a length of about 16 mm and a diameter of about 3.5 mm. In such a vibrator, in case of flexural vibrations the vibration nodes have a distance of about 3.6 mm from the ends of the vibrator. In the same filter the holding elements 2 have a diameter of 0.5 mm and a length of about 3.3 mm. The distance between the points of attachment of the elements 2 to the vibrator I and a vibration node may be 2.5 mm and the holding elements 2 may be situated between or outside the nodes. With these dimensions and the holding elements located outside the nodes a mechanical Q-factor of the vibrator of approximately 10 000 was measured for the desirable direction of vibrations. For the undesired direction of vibrations the resonance frequency of the vibrator was increased by 4 kc/s and the Q-factor derfis tg i$. ..QQ9;m

The vibrators consist of a hardened steel alloy with a high Young modulus and a low temperature coefficient, preferably of the well known NiSpanC. The material of the holding elements is not critical in respect to the temperature coefficient and may be normal steel.

It will be understood that the above description ofthe present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

I claim:

I. In a flexural vibrator for use as a resonator in a me chanical filter and exhibiting vibration nodes upon being flexurally vibrated and having at least one holding element connecting the vibrator to a base plate, the improvement wherein said holding element is connected to said vibrator at a substantial distance from the vibration nodes and is formed to constitute a quasi (2n l/4) flexural coupling line for the flexural vibrations established within said vibrator in a direction perpendicular to the longitudinal axis of said holding element at the center transmission frequency of the filter.

2. A flexural vibrator as defined in claim 1 wherein said holding element has a length less than one-quarter of the wave-length of longitudinal vibrations of said holding element at approximately the center transmission frequency of the filter.

3. A flexural vibrator as defined in claim 1 wherein said vibrator has a circular cross section.

4. A flexural vibrator as defined in claim 1 wherein said holding element has a circular cross section.

5. A flexural vibrator as defined in claim 1 wherein there are two spaced-apart holding elements. 

1. In a flexural vibrator for use as a resonator in a mechanical filter and exhibiting vibration nodes upon being flexurally vibrated and having at least one holding element connecting the vibrator to a base plate, the improvement wherein said holding element is connected to said vibrator at a substantial distance from the vibration nodes and is formed to constitute a quasi (2n1/4) lambda flexural coupling line for the flexural vibrations established within said vibrator in a direction perpendicular to the longitudinal axis of said holding element at the center transmission frequency of the filter.
 2. A flexural vibrator as defined in claim 1 wherein said holding element has a length less than one-quarter of the wave-length of longitudinal vibrations of said holding element at approximately the center transmission frequency of the filter.
 3. A flexural vibrator as defined in claim 1 wherein said vibrator has a circular cross section.
 4. A flexural vibrator as defined in claim 1 wherein said holding element has a circular cross section.
 5. A flexural vibrator as defined in claim 1 wherein there are two spaced-apart holding elements. 